The present invention generally relates to the preparation of semiconductor material substrates, especially silicon wafers, which are used in the manufacture of electronic components. More particularly, the present invention relates to a process for the treatment of Czochralski single crystal silicon wafers to dissolve existing oxygen clusters and precipitates, while preventing their formation upon a subsequent oxygen precipitation heat treatment.
Single crystal silicon, which is the starting material for most processes for the fabrication of semiconductor electronic components, is commonly prepared with the so-called Czoclualski process wherein a single seed crystal is immersed into molten silicon and then grown by slow extraction. As molten silicon is contained in a quartz crucible, it is contaminated with various impurities, among which is mainly oxygen. At the temperature of the silicon molten mass, oxygen comes into the crystal lattice until it reaches a concentration determined by the solubility of oxygen in silicon at the temperature of the molten mass and by the actual segregation coefficient of oxygen in solidified silicon. Such concentrations are greater than the solubility of oxygen in solid silicon at the temperatures typical for the processes for the fabrication of electronic devices. As the crystal grows from the molten mass and cools, therefore, the solubility of oxygen in it decreases rapidly, whereby in the resulting slices or wafers oxygen is present in supersaturated concentrations.
During the thermal treatment cycles typically employed in the fabrication of electronic devices, oxygen precipitate nucleation centers may form and ultimately grown into large oxygen clusters or precipitates. The presence of such precipitates in the active device region of the wafer can impair the operation of the device. Historically, to address this problem electronic device fabrication processes included a series of steps which were designed to produce silicon having a zone or region near the surface of the wafer which is free of oxygen precipitates (commonly referred to as a xe2x80x9cdenuded zonexe2x80x9d or a xe2x80x9cprecipitate free zonexe2x80x9d). Denuded zones can be formed, for example, in a high-low-high thermal sequence such as (a) oxygen out-diffusion heat treatment at a high temperature ( greater than 1100xc2x0 C.) in an inert ambient for a period of at least about 4 hours, (b) oxygen precipitate nuclei formation at a low temperature (600-750xc2x0 C.), and (c) growth of oxygen (SiO2) precipitates at a high temperature (1000-1150xc2x0 C.). See, e.g., F. Shimura, Semiconductor Silicon Crystal Technology, Academic Press, Inc., San Diego Calif.(1989) at pages 361-367 and the references cited therein.
More recently, however, advanced electronic device manufacturing processes such as DRAM manufacturing processes have begun to minimize the use of high temperature process steps. Although some of these processes retain enough of the high temperature process steps to produce a denuded zone, the tolerances on the material are too tight to render it a commercially viable product. Other current, highly advanced electronic device manufacturing processes contain no out-diffusion steps at all. Because of the problems associated with oxygen precipitates in the active device region, therefore, these electronic device fabricators must use silicon wafers which are incapable of forming oxygen precipitates anywhere in the wafer under their process conditions.
Accordingly, a process is needed by which existing oxygen clusters or precipitates in the silicon wafer may be dissolved, prior to the device fabrication, in such a way that future formation of oxygen precipitates within the wafer is prevented.
Among the objects of the invention, therefore, is the provision of a Czochralski single crystal silicon wafer, as well as the process for the preparation thereof, in which oxygen clusters and precipitates have been dissolved; and, the provision of such a wafer which will not form oxygen precipitates or clusters upon being subjected to an oxygen precipitation heat treatment.
Briefly, therefore, the present invention is directed to a process for heat-treating a Czochralski single crystal silicon wafer in a rapid thermal annealer to dissolve oxygen clusters, and to prevent future precipitate formation resulting from a subsequent thermal processing step. The process comprises heat-treating the wafer at a temperature of at least about 1150xc2x0 C. in an atmosphere having an oxygen concentration of at least about 1000 ppma to dissolve existing oxygen clusters and yield a wafer which is incapable of forming oxygen precipitates upon being subjected to an oxygen precipitation heat treatment.
The present invention is further directed to a process for heat-treating a Czochralgki single crystal silicon wafer to dissolve oxygen precipitates or clusters, and to prevent future precipitate formation resulting from a subsequent thermal processing step. The process comprises heat-treating the wafer in a rapid thermal annealer at a temperature of at least about 1150xc2x0 C. to dissolve existing oxygen clusters or precipitates, and controlling the cooling rate of the heat-treated wafer down to a temperature of less than about 950xc2x0 C. to produce a wafer which is incapable of forming oxygen precipitates upon being subjected to an oxygen precipitation heat treatment.
The present invention is still further directed to a process for heat-treating a Czochralski single crystal silicon wafer to dissolve oxygen precipitates or clusters, and to prevent future precipitate formation resulting from a subsequent thermal processing step. The process comprises heat-treating the wafer in a rapid thermal annealer at a temperature of at least about 1150xc2x0 C. in an atmosphere to dissolve existing oxygen clusters or precipitates. The heat-treated wafer is then cooled to a temperature between about 950 and 1150xc2x0 C. at a rate in excess of about 20xc2x0 C., and then thermally annealed at a temperature between about 950and 1150xc2x0 C. to produce a wafer which is incapable of forming oxygen precipitates upon being subjected to an oxygen precipitation heat treatment.
Other objects and features of this invention will be in part apparent and in part pointed out hereinafter.